Electronics Protection - November/December 2013 - (Page 8)

Feature Surge and Transient Protection for Telephone, CATV & Satellite Services Dave Perrotta, VP of Operations, Engineering & Manufacturing SurgeX Service Entrance Surge and Transient Protection Telephone, CATV and satellite services that enter commercial or residential premises are subject to surge and transient events in much the same way that the electrical service is. Direct and induced energy from lightning, as well as man made surges and transients can find their way onto these services and into the building, damaging or destroying connected equipment. The most common form of protection offered by the service providers is in the form of a shunt mode device such as a MOV, gas discharge tubes, carbon and other arc-through block that is put in line with the transmission or phone line and connected to earth. The theory being that when a surge event occurs, the device will divert the energy to earth ground once a certain break-down or arc-over threshold is reached. Electrical Service A ground rod is required by code at the power service entrance of a building. It, along with the service entrance and building wiring, provide the first line of defense against lightning entering the building. It does so by allowing surge energy to arc-over from the service entrance and building wiring to earth ground thus limiting the maximum voltage seen inside the building to around 6,000 volts. Proper Installation of Services For surge energy to properly get diverted to earth ground at the service entrance and protect connected equipment, it is imperative that all of the services enter the building at or very near the same place so they can all share a common earth grounding point. Unfortunately this is often not the case. It is quite common to see the electrical service enter a building at one location and be properly grounded, while the other services come in at different locations, usually dictated by convenience to, or ignorance by the installer. Once this happens, even if a ground rod is placed at the different service entrances or a less capable ground such as a water pipe is used, the rule of using a common grounding point for all services has been broken and problems will begin to occur. Multiple Earth Ground Sources Once different grounding points have been used in service installations, any surge current that gets properly diverted by the surge protection device enters the earth ground through a wire having some small amount of impedance. This impedance, although perhaps only a few tenths of an ohm when presented with many thousand amperes of surge current to divert, will cause the ground potential (what we normally think of as the zero voltage reference) to rise many hundreds or even thousands of volts at that point. 8 Shared Services If all services entering the building properly share a common earth ground at the service entrance the phenomenon described above would have no effect on connected equipment. All equipment would reference the same ground point and there would always be near zero volt difference between all equipment during a surge event. Such is not the case when different ground references are used and a surge event occurs. The many hundred or thousand volt shift in the ground reference due to having different ground sources is now present between the equipment that shares these services. With a different ground reference potential between equipment that is otherwise connected together with low- voltage telephone, CATV, network or audio wire, a completed circuit is created and the voltage that is present (usually several orders of magnitude greater than normal) causes damage or destroys the connected equipment. Line-level audio inputs/outputs on audio equipment, telephone PBX's, and network interface cards are particularly susceptible to this type of damage due to the much lower voltage and current signals present in normal operation. All-In One Surge Protection Devices In recent years, all-in-one surge protection devices have found their way into the marketplace. These devices range from inexpensive power-strips to high priced units that connect to the power, telephone, and CATV/satellite systems and claim to protect the connected equipment from surge and transient damage from any and all sources. These devices fail in three critical areas to protect equipment and proved a clean source of power. No Common Earth Ground Because these devices use shunt mode technology in the form of Metal Oxide Varistors (MOVs) to shunt the surge energy to ground and because they are not mounted at the service entrance, but rather sitting well inside the building and on a branch power circuit, they increase the problem of ground circuit contamination and ground loops. Any surge current that is successfully diverted to the ground wire inside the box must now travel some distance, usually several hundred feet through building wiring to the earth ground. This sets up that same ground-loop scenario as previously described, causing a voltage difference between connected equipment with subsequent damage. Redirection of Energy Anytime a surge event such as a lightning strike occurs it needs to be mitigated in the best possible manner before the majority of the energy can enter the building. If energy of any significant magnitude is allowed to enter the building though a service, the November/December 2013 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - November/December 2013

New Features Make F-Series TeraFrame Gen 3 One of CPI’s Most Advanced Cabinets
Silicones for High Reliability and Yield in Electronic Applications
Surge and Transient Protection for Telephone, CATV & Satellite Services
Thermal Management of LEDs: Looking Beyond Thermal Conductivity Values
Understanding NEMA Ratings for Electrical Enclosures
Silent Air Cooling: A New Approach to Thermal Management
VadaTech Releases Rugged Conduction-Cooled MicroTCA Ecosystem
Directable Inverted Blowers Deliver High Volume Air Flow
Cima NanoTech Launches Ultra Low Resistance Sante EMI Shielding Film
Littelfuse Introduces Surge-Tolerant Fuses
ProTek Devices’ TVS Array Provides Circuit Protection in Computing Applications
Reell’s PolyTorq Technology Expands Capabilities For Hinge and Torque Insert Applications
Industry News

Electronics Protection - November/December 2013